This application claims the benefit of Japanese Patent application No. 2001-319653 which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a liquid immersion type microscope objective lens, and more particularly to a liquid immersion type microscope objective lens suited to the use for, e.g., the patch clamp.
2. Related Background Art
In the biological field, a microscope is used often for the patch clamp as well as for morphological observations as conventionally performed.
The patch clamp is defined as a method for quantitatively measuring a change in membrane potential generated by action of an ion channel of a cell membrane.
A measurer puts a living cell into a culture liquid (medium) in a laboratory dish and brings a hyperfine electrode (glass electrode) into contact with a desired point of the cell in a way that observes it with a microscope.
Accordingly, the microscope objective lens for the patch clamp is classified as a liquid immersion type (the liquid immersion type microscope objective lens) of which a front end is contactable with the culture liquid, and is structured so that a gap (operation distance) between its front end and an object surface is taken sufficiently long enough to receive an insertion of the glass electrode.
Incidentally, the operation distance of the conventional liquid immersion type microscope objective lens for the path clamp is 2-3 mm (as exemplified in, for example, Japanese patent application Laid-Open No. 8-29237).
By the way, a magnification of the liquid immersion type microscope objective lens for the path clamp is approximately 60 at the maximum due to a restraint of the operation distance from being increased.
In recent years, however, a measurement target becomes hyperfine with an advancement of research, and hence there arise demands for increasing the magnification (up to, e.g., 100) of the liquid immersion type microscope objective lens for the path clamp and for increasing a numerical aperture.
Under such circumstances, it is an object of the present invention to provide a liquid immersion type microscope objective lens capable of increasing the magnification and the numerical aperture in a way that keeps a sufficient operation distance.
To accomplish the above object, according to one aspect of the present invention, a liquid immersion type microscope objective lens includes, in sequence from an object side, a first lens group containing a meniscus lens with the concave surfaces toward an object and having positive refractive power, a second lens group having positive refractive power, and a third lens group having negative refractive power, wherein a distance from an object surface to a first lens surface of the first lens group is larger than a focal length f of a whole objective lens system, and a radius of curvature r1 of the first lens surface in relation to the focal length f satisfies a relation such as 4 less than |r1/f| less than 7.
In the liquid immersion type microscope objective lens of the present invention, the first lens group is preferably constructed of a cemented meniscus lens and two or more single lenses.
In the liquid immersion type microscope objective lens of the present invention, it is desirable that a refractive index n3 of the single lens, closest to the object, of the single lenses satisfies a relation such as n3 greater than 1.65.
In the liquid immersion type microscope objective lens of the present invention, it is preferable that a radius of curvature r2 of a cemented surface of the meniscus lens and the radius of curvature r1 of the first lens surface satisfy a relation such as 0.6 less than |r2/r1| less than 0.9.